Many sound reinforcement applications require the accurate reproduction of live or recorded program material that has a wide frequency range, typically 40-18,000 Hz. Yet no single transducer practical for use in the art is capable of both accurately and efficiently reproducing this range of frequencies at high power levels. As a result, virtually all sound reinforcement systems, divide the program material into at least two separate frequency bands and provide a separate transducer subsystem for each band, optimized for the reproduction of its range of frequencies, the transducers preferably mounted in a common enclosure.
One such application is "stage monitoring" or "foldback".
A typical musical performance may include as many as fifty separate sound sources (both acoustic and electronic) onstage. While a sound reinforcement system is used to amplify and to adjust their relative levels for the audience, as many as eight essentially separate sound reinforcement systems are required to amplify and adjust their levels for the musicians onstage.
For a musician to maintain the correct vocal and/or instrumental pitch, timbre and tone, he must be able to hear himself. For him to maintain the correct musical relationship with the balance of the ensemble he must be able to hear them. In popular concerts, without a monitoring system, this is frequently difficult or impossible.
One reason is the disparity between various sources in acoustic energy; amplified instruments (such as electric guitars and keyboards) and even unamplified drums and brass will overwhelm nearby voices and acoustic instruments. A second reason is the wide distribution of these sources about the stage, which may place a source important for a musician to hear at a considerable distance from him. Third, only a fraction of the available sources may be relevant to any given musician.
A monitoring system is therefore required to amplify selectively for the musician those relevant sound sources whose location and/or limited acoustic energy would otherwise render them inaudible. Monitoring is also required to overcome distracting time-delayed reflections and reverberation from the auditorium. And, particularly in television, film, and theatrical productions, it may also be necessary for the performer to synchronize his actions with a prerecorded soundtrack.
There has, therefore, long been a demand for specialized sound reinforcement equipment for monitoring purposes, and particularly for suitable sound reinforcement enclosures both of minimal size and capable of generating the required high sound pressure levels.
Monitor enclosures of this type typically employ two 15" diameter cone-type loudspeakers (such as the JBL 2220) in an infinite-baffle or ported arrangement for frequencies below 1200-1600Hz, in combination with a horn-loaded compression driver (such as the JBL 2445J with 2385A) for frequencies above 1200-1600Hz. This produces an enclosure having a large frontal area and therefore consuming considerable floor space, which limits the locations on a crowded stage in which the monitor will fit; reduces the floor area available to the musician; and produces a less than desirable stage picture.
Despite considerable efforts devoted to the problem, no satisfactory method of reducing the size of monitor enclosures had heretofore been developed. Loudspeakers of smaller diameter have been substituted (for example, two 12" diameter loudspeakers for the 15" units), but only at the cost of low frequency power-handling. Similarly, the combination of a single 12" loudspeaker and a compression driver has been employed, but only at the cost of both power handling and low frequency response which severely limits the usefulness of such a monitor.
Existing monitor designs have other known drawbacks.
It is the object of a monitor enclosure to selectively amplify those sound sources of interest to the performer at whom it is aimed. To the degree that the enclosure radiates sound or "spills" beyond this area, it has a negative impact. This monitor spill will be picked up by additional microphones, muddying both the stage sound and that in the auditorium, as well as lowering the gain threshold at which feedback will occur. By increasing the distracting "noise floor" above which the adjacent musician must hear, monitor spill requires a compensating increase in the volume of his monitor, whose own increased spill initiates further rounds of escalation.
It is therefore a desirable object to control enclosure dispersion as a method of reducing monitor spill. The prior art use of infinite-baffle and ported designs for the cone loudspeakers provides little control over dispersion (as well as limited efficiency). Further, for these reasons, and because of the lack of projection typical of such designs, and the resulting rapid decay in volume with distance, such enclosures are limited to locations close to the subject. Horn-loading of the loudspeakers offers potential improvements in dispersion, efficiency, and projection but would also produce an unacceptable increase in enclosure height.
An additional drawback of prior art enclosures is the physical separation of the low-frequency and high-frequency sources, which, given the proximity of the listener, hampers intelligibility.
It is an object of this invention to disclose improved design techniques for loudspeaker enclosures generally, and for monitoring enclosures in particular, which allow the mounting of conventional components in an enclosure with an unprecedented reduction in both frontal area and total volume, while markedly reducing spill, improving projection, and increasing both efficiency and intelligibility.